In most turbofan engines, a lining is mounted between the engine casing and the first compressor stage or fan blade. The lining provides a tight clearance between the tip of the fan blade and the casing. In order to minimize the consequence of inadvertent contact between the rotating fan blade and the lining, the lining is formed from a material that may be abraded by the blade, and is often referred to as an abradable. An example abradable is disclosed in U.S. Pat. No. 5,655,701.
In the event the blade is struck by a foreign object, such as a bird entering the air intake of the engine, the fan blade may make a radial excursion coming into contact with the lining. As a result of the radial excursion, the lining is shorn by the fan blade tip. For a foreign object of significant size, up to 0.3" (0.8 cm) of the abradable may be shorn. This, in turn, significantly increases the blade tip clearance, and may cause air recirculation at the blade tip. As a result the fan blade may stall at its outer span, causing serious consequences to the engine, such as engine surges.
Often, engine casings include slots extending into the compressor section near the fan blan. These slots increase the clearance margin before the tip of the blade stalls (referred to as stall margin). However, these slots also reduce overall engine performance.
Accordingly, an improved lining, reducing susceptibility of the engine to consequences of radial excursions of a fan blade is desirable.